PREPARATION AND CHARACTERIZATION OF
Al-, Fe- AND Zr- PILLARED CLAYS AND THEIR
APPLICATIONS
by
Zehra MOLU
July, 2009 İZMİR
Al-, Fe- AND Zr- PILLARED CLAYS AND THEIR
APPLICATIONS
A Thesis Submitted to the
Graduate School of Natural and Applied Sciences of Dokuz Eylül University In Partial Fulfillment of the Requirements for the Degree of Doctor of Chemistry,
Chemistry Program
by
Zehra MOLU
July, 2009 İZMİR
OF Al-, Fe- AND Zr- PILLARED CLAYS AND THEIR APPLICATIONS” completed by ZEHRA MOLU under supervision of PROF. DR. KADİR YURDAKOÇ and we certify that in our opinion it is fully adequate, in scope and in quality, as a thesis for the degree of Doctor of Philosophy.
...
Prof. Dr. Kadir YURDAKOÇ
Supervisor
... ...
Prof. Dr. Melek MERDİVAN Prof. Dr. İ. Akın ALTUN
Thesis Committee Member Thesis Committee Member
... ...
Examining Committee Member Examining Committee Member
Prof. Dr. Cahit HELVACI Director
I would like to thank my supervisor, Prof. Dr. Kadir YURDAKOÇ for his guidance, his support and his critical suggestions throughout my doctoral studies. It was a privilege to study under his supervision.
I gratefully want to thank the committee of this dissertation, Prof. Dr. Melek MERDİVAN and Prof. Dr. İ. Akın ALTUN, for their comments and suggestions. They both brought unique perspectives to my research, enriching it greatly. I would also like to thank Prof. Dr. Mürüvvet YURDAKOÇ for her comments and valuable guidance.
I gratefully acknowledge the monetary and moral support of Scientific and Technical Research Council of Turkey (TUBİTAK) -Münir Birsel Foundation for the Doctorate award.
I’m also grateful to Research Foundation of Dokuz Eylül University (Project No: 2007.KB.FEN.028) for financial support.
Very special thanks would be given to my parents, Fatma and Hüseyin Bekçi, and my brother, Ethem, for their forever love, confidence in me and understanding.
Finally, I would like to express my special gratitude to my dear husband, Cafer Molu, for his love, endless support, understanding and sacrifices. He always gives me strength to carry on…
ABSTRACT
Aluminum, iron, zirconium - pillared clays were prepared by using Montmorillonite KSF and K10 and used as inorganic fillers for synthesis poly(acrylic acid/ Aluminum and zirconium pillared clays) superabsorbent composites. Pillared clays and Al- KSF, Al-K10 and Zr-KSF based superabsorbents were characterized by XRD, FTIR, gas adsorption analysis, SEM and TGA. Pillaring process resulted in a strong increase in the d001 basal spacing. BET surface area analyses indicated that the surfaces areas of pillared
clays were increased. The superabsorbents showed pH dependent and reversible swelling behaviors. Decreasing of swelling ratio was observed by increasing the content of Zr-KSF on the superabsorbent. The more crosslinking ratio of Al-K10 based superabsorbents was understood from less swelling character than Al-KSF based superabsorbents. XRD revealed that the layers of clay dispersed on the composite. FTIR analyses indicated that the shifts of stretching vibrations of O-Si-O tetrahedra and OH bands supported ester formation between acrylic network and pillared clay. Additionally, the efficiency of aluminum-pillared-layered K10 and KSF montmorillonites (Al-PILCs) for the removal of Trimethoprim (TMP) was investigated. The removal percentage was highly depended on pH. TMP and the adsorbent surface, both carrying positive charges at acidic pH values may explain the poor adsorption. Al-K10 exhibits higher removal capacity of TMP at lower adsorbent dosages in comparison with Al-KSF. Adsorption is increased by increasing temperature onto Al-KSF and increased by decreasing temperature onto Al-K10. From Dubinin–Radushkevich isotherm equation, the adsorption onto Al-KSF and Al-K10 can be explained by ion-exchange mechanism and physical adsorption type, respectively. Standard enthalpy value of the sorption indicates that adsorption is endothermic while the standard entropy suggests that randomness increases during adsorption.
superabsorbent composites.
ÖZ
Alüminyum, demir, zirkonyum–sütunlu killeri Montmorillonit KSF ve K10 kullanılarak hazırlandı ve poli(akrilik asit/ Alüminyum ve Zirkonyum sütunlu killeri) süperabsorplayıcı kompozitlerde inorganik dolgu maddesi olarak kullanıldı. Sütunlu killer ve süperabsorplayıcılar XRD, FTIR, gaz adsorpsiyon analizi, SEM ve TGA ile karakterize edildi. Sütunlama işlemi d001 bazal aralıkta büyük bir artışla sonuçlandı. BET
yüzey alanı analizleri sütunlu killerin yüzey alanlarının arttığını gösterdi. Süperabsorplayıcıların pH’a bağımlı ve tersinir şişme özelliğine sahip oldukları yapılan şişme deneyleri ile anlaşıldı. Süperabsorplayıcıdaki Zr-KSF’nin miktarı artmasıyla şişme oranında düşme gözlenmektedir. Al-K10 tabanlı süperabsorplayıcının Al-KSF tabanlı süperabsorplayıcıya göre daha çok çapraz bağlanma yaptığı daha az şişme özelliği göstermesinden anlaşılmaktadır. XRD sonuçları kilin tabakalarının kompozit içinde dağıldığını ortaya çıkarmıştır. FTIR analizleri, O-Si-O tetrahedral gerilmedeki ve OH bandlarındaki kaymaların akrilik yapı ile sütunlu kil arasında ester oluşumunu desteklediğini göstermektedir. Bunlara ek olarak, bu çalışma alüminyum- sütunlanmış-tabakalı K10 ve KSF montmorillonitlerin (Al-PILC’ler), Trimetoprimin (TMP) sulu çözeltiden uzaklaştırılmasında etkinliğini araştırmıştır. TMP’nin uzaklaştırılma yüzdesi büyük oranda pH’a bağlıdır. TMP ve adsorplayıcı yüzeyinin her ikisinin de asidik pH değerlerinde pozitif yük taşıması, Al-PILC’lerin yüzeylerine TMP’nin daha zayıf tutunmasını açıklayabilmektedir. Al-K10, Al-KSF ile kıyaslandığında daha düşük adsorplayıcı miktarında daha yüksek TMP’i uzaklaştırma kapasitesi göstermektedir. TMP’nin Al-KSF üzerine olan adsorpsiyonu sıcaklık artarken, Al-K10 üzerine olan ise sıcaklık düşerken artmaktadır. Dubinin–Radushkevich izoterm denkleminden, Al-KSF ve Al-K10 üzerine olan adsorpsiyon sırasıyla iyon-değişimi ve fiziksel adsorpsiyonla açıklanabilir. Standart entalpi değeri adsorpsiyonun endotermik olduğunu gösterirken standart entropi değeri de düzensizliğin adsorpsiyon sırasında arttığını belirtmektedir.
dolgu maddesi olduğunu sonuçlar göstermektedir.
Anahtar sözcükler: Sütunlu kil killer, hidrojel, süperabsorplayıcı kompozit, adsorpsiyon
Page
Ph.D.THESIS EXAMINATION RESULT FORM...ii
ACKNOWLEDGEMENTS ...iii
ABSTRACT ...iv
ÖZ...vi
CHAPTER ONE – INTRODUCTION...1
1.1 Clays and Catalysts ... 1
1.2 Clays………... 1
1.2.1 Structure of the Clay Minerals ... 2
1.2.1.1 Smectite [2:1 or TOT structure]... 5
1.3 Pillared Clays …………..………... 7
1.3.1 A Historical Perspective …...………. 7
1.3.2 Pillaring ………...……….. 8
1.3.3 Catalytic Applications of PILCs ………...………... 10
1.3.3.1 Suface Acidity–Dependent Application ………...…………... 10
1.3.3.2 Catalytic Active Substrates- Dependent Applications ………...……. 10
1.3.4 The Importance of the Surface Acidity ………...……… 11
1.3.5 The importance of Porous Structure ………...………. 12
1.3.6 Aluminum Pillared Clays (Al-PILC) ………...……… 12
1.3.7 Zirconium Pillared Clays (Zr-PILC) ………...……… 14
1.3.7.1 Influence of pH on Preparation of Zr-PILC ………. 15
1.3.7.2 Influence of concentration on Preparation of Zr-PILC …………...… 15
1.3.7.3 Influence of temperature and time on Preparation of Zr-PILC…….... 16
1.3.7.4 Effect of Drying Conditions on the Structure of Zr-PILC ………... 16
1.4.1 Adsorption Processes …………...………...………. 22
1.4.2 Adsorption Isotherms ………...………...… 24
1.4.2.1 Langmuir Isotherm ………...………... 25
1.4.2.2 Freundlich isotherm …...……….. 28
1.4.2.3 Dubinin – Radushkevich (DR) Isotherm ………...……….. 29
1.4.3 Adsorption Kinetics ………...……….. 30
1.4.4 Thermodynamic Parameters of Adsorption ………...……….. 32
1.5 Trimethoprim ………...……… 34
1.6 Characterization Techniques of Pillared Clays ...……….... 36
1.6.1 The BET Analysis ………...……….... 36
1.6.2 X-Ray Powder Diffraction ..………...………. 37
1.6.3 Thermal Analysis (TGA/DTG) ..……...……….. 38
1.7 Preparation, characterization and swelling behaviors of poly (acrylic acid)/Pillared clays, superabsorbent composites...………... 39
1.8 Objectives and Scope of the Thesis Study ………...………... 40
CHAPTER TWO – MATERIAL AND METHOD...42
2.1 Properties of clay samples ……..………...………..… 42
2.2 Preparation of Aluminum pillared clays (Al- PILCs) ……...……….. 42
2.3 Preparation of Zirconium pillared clays (Zr- PILCs) ……...………...… 43
2.4 Preparation of Iron pillared clays (Fe- PILCs) …………...………. 43
2.5 Synthesis of poly (acrylic acid)/Pillared clays, superabsorbent composites ...… 44
2.6 Swelling measurements of poly (acrylic acid)/Pillared clays, superabsorbent composites ……….…... 45
2.7 Characterization Techniques ………...……….... 45
2.7.1 XRD analysis …...………..….. 45
2.7.5 SEM Measurements ……...……….. 47
2.8 Adsorption experiments …………...……… 47
CHAPTER THREE – RESULTS ………... 49
3.1 Characterization of pillared clays ……...………. 49
3.1.1 XRD analyses of pillared clays …………...………...………. 49
3.1.2 FTIR analyses of pillared clays ………...……… 56
3.1.3 SEM analyses of pillared clays ……….………... 66
3.1.4 N2 adsorption–desorption isotherms of pillared clays …...……….. 73
3.1.2 TGA analyses of pillared clays ………...………. 81
3.2 Characterization and swelling behaviors of poly (acrylic acid)/Aluminium pillared Montmorillonite K10 and KSF ………..………... 88
3.2.1 XRD analysis of Al-KSF and Al-K10 based superabsorbents ...…...…….. 88
3.2.2 FT-IR analysis of Al-KSF and Al-K10 based superabsorbents ….…..…... 91
3.2.3 SEM images of Al-KSF and Al-K10 based superabsorbents …...………... 93
3.2.4 Swelling capacity of Al-KSF and Al-K10 based superabsorbents …...…... 94
3.2.5 pH-sensitivity of Al-KSF and Al-K10 based superabsorbents …...………. 95
3.2.6 Swelling reversibility of Al-KSF and Al-K10 based superabsorbents ….... 96
3.3 Sorption experiments of Aluminium pillared Montmorillonite K10 and Montmorillonite KSF ………..………...……….… 97
3.3.1 Characterization of Al pillared clays for adsorption study …...…………... 97
3.3.2 Adsorbent dose effect ………...………. 105
3.3.3 pH effect on adsorption …………...……….… 106
3.3.4 Effect of contact time on adsorption ………. 108
3.3.5 Adsorption kinetics ……...………...……….. 109
3.3.6 Adsorption isotherms………...………... 114
3.4.1 Swelling rate measurements of Zr-KSF based superabsorbents ……...… 121
3.4.2 pH-sensitivity of Zr-KSF based superabsorbents ………..………... 122
3.4.3 Swelling reversibility of superabsorbents ………...…………..… 124
3.4.4 SEM images ………..……… 125
3.4.5 FTIR Analysis ………...……….... 127
3.4.6 XRD analysis …...……….. 128
CHAPTER FOUR– CONCLUSION ……… 130
4.1 Conclusion ………. 130
INTRODUCTION
1.1 Clays and Catalysts
The main properties of cationic and anionic clays as well as their role in catalysis are discussed in light of the versatility and potential usage of these materials. Clays exhibit specific features such as high versatility, wide range of preparation variables, use in catalytic amounts, ease of set-up and work-up, mild experimental conditions, gain in yield and or selectivity, low cost, etc., which may be very useful tools in the move towards establishing environmentally friendly technologies. Furthermore, the possibility of upgrading these materials by the pillaring process opens new and interesting perspectives, also considering possible shape selective advantages. Recent catalytic applications of cationic and anionic clays in organic or fine chemistry (acid- or base-catalyzed reactions, Diels–Alder reactions, reactions using metallic nitrates, etc.) ,environmental catalysis (DeSOx, DeNOx or contaminant oxidation) and energy exploitation (partial oxidation of methane) are discussed as very promising research subjects with a wide range of possible future developments with the transition from 2- 3- dimensional structures via the preparation of pillared clays (Vaccari, 1998, s.161).
1.2 Clays
Clays are very versatile materials and hundred of millions of tons currently find applications not only in ceramics and building materials, paper coatings and fillings, drilling muds, foundry moulds, pharmaceuticals, etc, but also as adsorbents, catalysts or catalyst supports, ion exchangers, etc, depending on their specific properties (Grim, 1968; Van Olphen, & Fripiat, 1979; Nemecz, 1981; Fodwen et al., 1984; Reichle, 1986; Newman, 1987; Schoonheydt, 1991; Cavanie et al., 1991).
“Clay minerals” refers collectively to the group of fine- grained hydrous silicates of aluminum (and also to some extent of magnesium or iron). When examined under the scanning electron microscope, these minerals are seen to consist of readily identifiable particles, which can have a variety of geometric shapes. Despite this variety of morphology, clays are closely interrelated in terms of their basic crystal structures, and also in the characteristic physical and chemical properties resulting from their crystal chemistry (Wilson, 1987).
1.2.1 Structure of The Clay Minerals
Most clay minerals involve two basic types of units in their atomic structure. The manner in which these units are combined and the type of exchangeable atoms that are present will play a very significant role in the type of clay mineral that forms.
The first unit is built of silica tetrahorns. The tetrahedral sheet is composed of silicon bounded to four oxygen atoms. Each unit consists of a central four-coordinated atom (e.g. Si) surrounded by four oxygen atoms that, in turn, are linked with other nearby atoms (e.g. Si), thereby serving as inter-unit linkages to hold the sheet together (Juma, 1998).
Figure 1.1 Silica tetrahedral units Figure 1.2 Tetrahedral Sheet
The second unit consists of octahedrals. An octahedral sheet, consisting of two layers of oxygen atoms or hydroxyl groups, between which aluminium, magnesium, iron are bonded in six – fold coordination. If the layer contains mainly trivalent aluminium ions which occupy two thirds of the available octahedral interstices, this is known as a gibbsite type or dioctahedral layer. Conversely, if the layer contains mainly divalent magnesium ions, this will occupy all of the octahedral sites, and the layer is called a brucite – like or trioctahedral layer (Wilson, 1987).
Figure 1.4 Octahedral unit Figure 1.5 Octahedral sheet Figure 1.6 Octahedral sheet
The structural units of clays therefore consist of either alternating tetrahedral (T) and octahedral sheets (O) (OT or 1:1 structure), as in the kaolinite group of clay minerals; tetrahedral sheets (TOT or 2:1 structure), as in illite and the smectite clay minerals, of which the most common member is montmorillonite; or an arrangement in which the layer TOT units alternate with a brucite layer (2:1:1 structure) as in chlorite. The basic structural arrangements of the more common clay minerals are illustrated schematically in Figure 1.7 (Wilson, 1987).
Figure 1.7 Layer structures of two-layer minerals (1:1 clay minerals) (A) and of three-layer minerals (2:1 clay minerals)
(B); T, O=tetraeder and octaeder layers, respectively; dL=layer distance.
Table 1.1 Clay minerals and their net charge
Group Layer Type Net Negative Charge (Coulombmolkg-1) Surface Area (m2/g) Basal Spacing (nm) Kaolinite 1:1 2-5 10-30 0.7 Fine-grained mica 2:1 15-40 70-100 1.0 Smectite 2:1 80-120 600-800 1.0-2.0 Vermiculite 2:1 100-180 550-700 1.0-1.5 Chlorite 2:1:1 15-40 70-100 1.4
1.2.1.1 Smectite [ 2:1 or TOT structure]
Smectite minerals have three layers with the aluminum atoms lying between two layers of silicon atoms in a 2:1 structure, sharing the valencies of their oxygen atoms (Juma, 1998).
This group is composed of several minerals including pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite and montmorillonite. They differ mostly in chemical content. Montmorillonite, a type of smectite is expanding clay mineral and different layers are held together by bonding between divalent cations and water with basal oxygen atoms of the tetrahedral sheets. The formula for montmorillonite is (Si7.8Al0.2)(Al3.4Mg0.6)O20(OH)4. The formula indicates that there is substitution for Si4+
by Al3+ in the tetrahedral layer and for Al3+ by Mg2+ in octahedral layer (Juma, 1998).
These substitutions lead to net negative charges on the clay structure which must be satisfied by the presence of charge-balancing cations somewhere else in the structure. The interlayer is hydrated, which allows cations to move freely in and out of the structure. Because the interlayer is open and hydrated, cations may be present within the interlayer to balance negative charges on the sheets themselves. These cations between the layers are part of the cation exchange capacity of the soil. Smectites will have a CEC of around 80 to 150 meq/100 g. (Juma, 1998). Also, the amount of water present in the interlayer of montmorillonite results in swelling under wet conditions and shrinking in dry conditions (Juma, 1998).
Smectites are swelling clays, thus water is able to enter into the interlayer region, thereby expanding the clay layers, and the hydrated cations can be exchanged with other type of larger hydrolysed metal cations or organic/ inorganic complexes. These two properties of smectites, swelling and ion-exchangeability are critical for the successful synthesis of PILCs (Pillared Clays).
Figure 1.8 Idealised structure of a smectite clay mineral showing the
(Ο) Oxygen atoms and (•) hydroxyl groups
Therefore, the common procedure for the preparation of PILC materials is considered to be: (i) swelling of smectite in water; (ii) exchanging the interlayer cations by partially hydrated polymeric or oligomeric metal cation complexes in the interlamellar region of the clay; (iii) drying and calcining of the wet cake of expanded clay to transform the metal polyoxocations into metal oxide pillars, which would 1) open up the silicate platelets permanently and 2) form covalent bonds with the tetrahedral sheets of the clay. So, in step (ii) it is still possible to exchange the pillars, while in step (iii), the pillars are permanently bonded to the clay layers (Ding, Kloprogge, & Frost, 2001).
The main properties relevant for catalytic applications are affected considerably by the composition and particle size (Grim, 1968). As a function of their small particle size, clays may exhibit both Bronsted and Lewis acid sites: the former are the external OH -groups, while the Lewis sites are the exposed or three-fold coordinated Al3+ ions, substituting for the Si4+ ions in the tetrahedral sheets. The strength of the Bronsted sites may be determined by Hammet indicators, butylammine titration or IR spectroscopy using probe molecules (Newman, 1987; Schoonheydt, 1991; Knozinger, 1993), with a direct correlation between acid strength and composition. The surface acidity decreases as the amount of residual water in the clay increases, with an extension related to the nature of the exchangeable cations (Schoonheydt, 1991).
Not only acid sites, but also electron-accepting or oxidizing sites may be located at the edges or in the structure. The former may be identified as trigonal Al3+ ions, acting as Lewis sites, while Fe3+ ions in the lattice are the structural oxidizing sites (Theng, 1974). Moreover, redox properties may be induced by the exchangeable cations, such as Cu2+ , Ag+ , Fe3+ or Ru3+ (Y. Soma, M. Soma and I. Harada, 1986).
1.3 Pillared Clays
1.3.1 A Historical Perspective
Barrer and McLeod (1995) demonstrated the concept of intercalation of clays by organic compounds. However, organic and organometallic intercalating or pillaring agents decompose at relatively modest temperatures causing the pillared clay structure to collapse.
Nowadays, these types of pillared clays are industrially used as gelling agents, thickeners, and fillers (Schoonheydt, 1991).
The escalation of the oil prices in 1973 confronted the oil industry with the problem of how to maximize the processing of crude oil, especially the heavy fractions to gasoline components (Ding, Kloprogge & Frost, 2001). The problems then cascaded to fluid catalytic cracking (FCC) catalyst design and process development. The inherent problem with zeolite catalysts in processing such material is the relatively small pore size of zeolites, and the large amount non-selective pre-cracking that would have to take place before the large reside molecules were reduced to a size capable of diffusing into the very active and selective zeolite component of the catalysts (Smith & Dytrych, 1984; Meier, 1986). A strong impetus was thus given to the development of catalysts with relatively large pore sizes, able to deal with larger molecules than the existing molecular sieves, and good thermal and hydrothermal stability. The oil crisis thus resulted in a renewed interest in the concept of pillared clays. The use of inorganic hydrated
polyoxocations as pillaring agents provided thermally stable pillared clays with high specific surface areas (200 to 500 m2/g). Upon calcination the hydrated polyoxocations dehydrate and dehydroxylate, and react to form fixed metal oxide pillars.
Figure 1.9 Schematic structures of the single and mixed metal oxide PILCs with various d-spacings. (a) Al-PILC; (b) Si/Ti-PILC.
1.3.2 Pillaring
Pillared layered clays PILC’s, nanocomposite materials with open and rigid structures are obtained by linking robust, three-dimensional species to a layered host. The final properties of PILC’s can be modulated by carefully choosing the different parameters, such as nature of the pillaring agent, type of clay and particle size, pillaring procedure, thermal treatments, etc., thus offering a very powerful and flexible way to design tailored catalysts. Therefore, control of the pillaring process is a very promising means to obtain solids with i) very high surface areas (up to 600 m2g-1), ii) a broad spectrum of properties structural, chemical, catalytic, etc., and iii) controlled internal structures, with reactive sites and/or species chosen to match particular applications or provide host structures for chemical or physical processes (Vaccari, 1999).
Many different pillaring agents have been reported in the literature organic compounds, metal trischelates, organometallic complexes, metal cluster cations, metal oxide sols, polyoxocations, etc. Many of these species however have some drawbacks, such as low reactivity or lack of thermal stability. Polyoxocations are by far the most
widely employed pillaring agent. Different polyoxocations (Al, Ni, Zr, Fe, Cr, Mg, Si, Bi, Be, B, Nb, Ta, Mo, Ti and more recently Cu and Ga) have been reported in the open and patent literature and clays with multielement pillars also have been prepared (Schoonheydt, 1991; Vaughan, D.E.W. 1987).
Preparation of PILC’s consists in a controled hydrolysis reaction which can be carried out in solution or in the interlamellar space of the clay. The calcination process also plays a key role. Three general cases may occur: i) the polyoxocations exit from the clay (no pillaring) , ii) they degrade in situ giving rise to layers of aluminum hydroxide (1.4 nm thick, corresponding to a pseudo-chlorite ), and iii) in the case of true pillaring, the polyoxocations dehydrate up to 573 K and dehydroxylate between 573 and 673 K. At higher temperatures, the pillars transform progressively, but the clays maintain the initial spacing of about 1.9 nm, while at T >1073–1173 K the clays degrade (Vaccari, 1999). The stability of pillared clays as a function of the calcination temperature depends on the nature of the clays and the composition of the pillars. For example, an increase in stability may be achieved using mixed polyoxocations or by doping with small amounts of another element (Vaughan, 1987; Carrado, Kostapapas, Suib,& Coughlin, 1986 & Occelli, 1986).
From the viewpoint of making catalysts at a competitive price, three important criteria need to be met: i) use the whole clay material, after minimal refining (i.e., low cost) , ii) pillar the Ca or (Ca, Na) forms, not only the Na forms (i.e., no pre-exchange), and iii) be able to use clay-polyoxocation concentrations (>15% solids) that can be economically and effectively spray-dried to give a usable particle size distribution (40– 200 µm) (Burch & Warburton, 1987).
1.3.3 Catalytic Applications of PILCs
1.3.3.1 Suface Acidity–Dependent Application
PILCs are used in catalytic cracking process, and also PILCs find application as acid catalysts in the synthesis of chemicals. Different types of reactions have been studied, such as hydroisomerisation (Moreno, Kou, & Poncelet, 1996, 1997; Moreno, Gutierrez, Alvarez, Papayannakos, & Poncelet, 1997; Moreno, Kou, Molina, & Poncelet, 1999), dehydration (Jones & Purnell, 1994), dehydrogenation (Lourvanij & Rorrer, 1997), hydrogenation (Louloudi & Papayannakos, 1998), aromatisation (Liu, Zhao, Sun & Min, 1999), disproportionation (Chevalier, Franck, Suquet, Lambert & Barthomeuf, 1994), esterification (Wang & Li, 2000), alkylation (Geatti, Lenarda, Storaro, Ganzerla & Perissinotto, 1997), etc. Yang and co-workers have published a series of reports on SCR of NO to nitrogen with either ammonia or hydrocarbon as reductants (Yang, Chen, Kikkinides, Cheng & Cichanowicz, 1992).
1.3.3.2 Catalytic Active Substrates- Dependent Applications
Modification of PILCs by metal deposition can be easily achieved through impregnation methods (Wang & Li, 2000). These metals, such as Pt, Ni, Cu, etc., are generally active for a variety of catalytic reactions. In addition, some types of metal oxides in PILCs are also catalytically active. Therefore, apart from exploiting the surface acidity and porous structure of the PILC in catalytic applications, it is of great interest to take advantages of these catalytically active substrates. In fact, all these three characteristics, surface acidity, porous structure and catalytic active substrates, would have correlative effects on the catalytic performance of PILCs and one of them will generally be dominant.
Loading a PILC with transition metal ions, because of the oxidation state of these metal ions, will lead to new materials that have the potential to be applied as a catalyst in
redox reactions. In recent years, complete mineralization of organic pollutants in water, termed advanced wastewater treatment, had drawn more and more attention (Liu, Liptak, & Bouis, 1997; Baird, 1998). In this process, the organic compounds are oxidized to carbon dioxide and water, thereby preventing secondary pollution. The most popular methods for this process include homogeneous photocatalytic oxidation (Legrini, Oliveros & Braun, 1993), wet air/peroxide oxidation (Lei, Hu, & Yue, 1998), heterogeneous photocatalytic oxidation (Ollis, Pelizzetti & Serpone, 1991) and catalytic wet air/peroxide oxidation (Schiavello, 1997). The first two methods employ only ozone or hydrogen peroxide as oxidant, while the latter two methods involve heterogeneous catalysts. The most popular catalyst for heterogeneous photocatalysis is TiO2 (Serpone,
1995), while for catalytic wet oxidation, Cu or Fe loaded on particle supports is widely investigated (Barrault, et al., 1998). Therefore, it is interesting to utilize PILC as a support for these active substrates. However, only limited research has been conducted in this area.
1.3.4 The Importance of the Surface Acidity
PILCs possess both Bronsted acid (proton donor) sites and Lewis acid (electron pair acceptor) sites (Chevalier & et al., 1994). To investigate the surface acidity on PILC samples, the most commonly used methods are the study of adsorbed ammonia or pyridine by infrared (IR) spectroscopy and temperature programmed desorption (TPD) of adsorbed ammonia and amine titration using Hammett indicators. By choosing adsorbed substrates with different basicity, both the number and strength of the acid sites can be determined. It is reported that 2,6-dimethylpyridine (DMPY) is selectively adsorbed on Brønsted acid sites, thus it can be used as a good probe molecule for the determination of Brønsted acidity in clays and PILCs. Generally, it is believed that Brønsted acidity is mainly coming from the clay layer structural hydroxyl groups, while Lewis acidity is attributed to the metal oxide pillars. In addition, the amount and strength of Brønsted and Lewis acid sites are closely related to the types of clays and metal oxide pillars.
Apart from the types of starting clay materials and metal oxide pillars, acid-activation of clays before pillaring is widely accepted as another feasible way to improve the acidity, particularly Brønsted acidity. In addition, such acidity enhancement always coupled with the increase of the pore volume and average pore diameter (Mokaya, Jones, Davies, & Whittle, 1993).
1.3.5 The importance of Porous Structure
The pore size of PILCs can be varied from 5 Å to over 20 Å, depending on the synthesis conditions, such as type of the starting clay materials, cation exchange capacity (CEC) of clays, type of the metal oxide pillars, and thermal treatment temperature. Recent reports show d-spacing as high as 60 Å can be obtained on some composite PILCs, such as Si/Ti-PILC (60 Å), Si/Cr-PILC (47 Å), Si/Fe-PILC (63 Å) (Han, Matsumoto & Yamanaka, 1997) etc. In general, the porous structure of PILCs can be stable up to 673–773 K. Further increase the heating temperature will lead to the collapse of the clay layers because of the sintering of the pillars and the dehydroxylation of the clay sheets. Both the nature of the clay materials and type of metal oxide pillars will affect the thermal stability of the resulting PILCs. It has been reported that a homogeneous distribution of pillars is beneficial for improving the thermal stability. Katdare and co-workers showed that an Al-PILC prepared by a simplified method with ultrasonic modification had superior thermal and hydrothermal stability (Schiavello, 1997).
1.3.6 Aluminum Pillared Clays (Al-PILC)
Pillared clays have reached considerable interest as catalysts and catalysts supports over the past years. Their porosity, reactivity and thermal stability are being widely applied in adsorption and catalysis (Kikuchi, & Matsuda, 1988; Mrada, Ghorbela, Tichit, & Lambertc, 1997). Metals commonly used as ionic precursors are Al, etc. Aluminium
has been profusely used as pillaring agent due to its hydrolisis capability to form large inorganic polymeric oxy-hydroxy cations of several nuclearities. The major aluminium species used is the [Al13]7+ polymer but other mono and polynuclear species may be
present in solution depending on its pH (Lambert, & Poncelet, 1997). The actual exchange is dependent, as in every diffusion process, on the pillaring agent solution concentration and the ageing conditions of the formed cation whose size and charge (n/q ratio) deeply depends on the synthesis conditions. Also, the effect of the drying method is claimed to be more important than the pillaring reagent itself or even the clay layer charge on PILCs pore opening, giving rise to products with uniform pore size when air-dried or with a continuous distribution of larger pores when freeze-air-dried. With all that in mind, the possibility of tuning the properties of PILCs by varying the synthesis conditions seems plausible (Hutson, Hoekstra, & Yang, 1999; Flegoa, Galassoa, Millinia, & Kirics, 1998).
In general, the pillaring processes are carried out in diluted systems, less than 2 wt.% of clay suspension and less than 0.5 M of pillaring solution (Figueras, 1988; Lambert, & Poncelet, 1997). When a large amount of materials has to be prepared, a quite large volume of solutions has to be handled which turns the PILCs synthesis in a non-viable industrial method. Vaugan (Vaugan, 1988) proposed the first approaches to prepare PILCs in large quantities. More recently, a method of synthesis based on the use of highly concentrated clay suspensions without any previous purification or homoionisation of clay prior to pillaring has been developed (Storaroa, Lenardaa, Gazerlaa, & Rinaldi, 1998). The method enables the preparation of industrial quantities of pillared materials with uniform properties, and its scale-up seems promising. However, only scarce information has been, up to the moment, reported on the physico-chemical characteristics of the resulting materials and on their comparison with those obtained by the more conventional method of diluted suspensions (Salerno, Asenjo, & Mendioroz, 2001).
The first step in the pillaring process is to prepare a pillaring agent. In the case of the Al13 polyoxocation, two methods are commonly used: (1) mixing of aqueous AlCl3 with
Al to form a chlorohydrate which is also commercially available, and (2) addition of a base to AlCl3 or Al(NO3)3 solutions with OH/Al3+ ratios up to 2.5. The polyoxocation
complex produced has been analyzed and is thought to be the tridecamer [AlO4Al12(OH)24(H2O)12]7+, also referred to as the Keggin ion (Kloprogge, 1998;
Kloprogge, Seykens, Geus, Jansen, 1992; Kloprogge, & Frost, 1999).
Figure 1.10 Structure of the Al13 complex.
The next step is the mixing of a clay suspension with this polyoxocation solution. This allows the interlayer cations in the clay to exchange with the polyoxocation in solution through cation exchange reaction or intercalation (Gil, Gandia, 2000). After the intercalation process is complete, the clay is separated, washed and then calcined. The property of the stable pillared structure obtained is greatly affected by factors such as clay used, mixing and drying conditions and polycation/s used.
1.3.7 Zirconium Pillared Clays (Zr-PILC)
Several experimental parameters (ageing temperature, ageing time, pH, and concentration) can affect the degree of polymerization of hydroxyl zirconium species in
aqueous solution. The solution chemistry of Zr is complex, but it is known that zirconyl ion is present in solid zirconium chloride as the tetramer, [(Zr (OH)2.4H2O)4) ]8+
(Clearfield & Vaughan, 1956). The four Zr ions are located at the corners of a slightly distorted square and are linked together by OH bridges above and below the plane of the square. When dissolved in water the solution becomes quite acidic due to hydrolysis of the tetramers. Further hydrolysis eventually leads to a tetramer with the formula [Zr4(OH)14.(H2O)10]2+. Here, two of the zirconium ions have a single positive charge and
the other two are neutral. On ageing the solution larger species are observed (Muha, & Vaughan, 1960) which are believed to be due to a polymerization reaction in which a neutral site on one tetramer reacts with a singly charged site on another tetramer, the result being that the zirconium ions are connected by two OH bridges. Continuation of this process leads to the formation of large “rafts” or two-dimensional hydroxy polymers. Further polymerization eventually results in the precipitation of hydrous Zr(OH)4 (Ross, 1988).
1.3.7.1 Influence of pH on Preparation of Zr-PILC
Increasing of the pH of the solution, by addition of NaOH for instance, favours the hydrolysis reaction and this in turn leads to an increase in the degree of polymerization as the number of neutral zirconium ions increases. Precipitation of hydrous Zr(OH)4
begins at a pH of about 3 (Vaughan, Lussier, & Magee, 1979) give an example of a preparation of a Zr-PILC in which the zirconyl chloride solution is treated with Na2CO3.
This will increase the degree of polymerization and lead to polymeric hydroxyl zirconium pillars (Ross, 1988).
1.3.7.2 Influence of concentration on Preparation of Zr-PILC
A solution of lower concentration will have a higher pH and so the degree of polymerization will be higher. Concentrations typically used in the preparations described in the earlier literature are in the range 0.1 – 0.33 (Ross, 1988).
1.3.7.3 Influence of temperature and time on Preparation of Zr-PILC
Heating of the zirconyl chloride solution also leads to an increase in the degree of polymerization, as demonstrated by Clearfield, & Vaughan, 1956. This is simply due to an increase in the rate of hydrolysis with temperature. Similarly, increasing the ageing time at any specific temperature will increase the degree of polymerization.
Surface areas obtained are critically dependent on the method of preparation. Samples synthesized from refluxed Zr solutions had markedly higher surface areas than those prepared using fresh, unrefluxed Zr solutions. As already mentioned, the effect of refluxing is to increase the degree of polymerization. Hence, as might be expected, it appears that an increase in the degree of polymerization produces an increase in the surface area of pillared clay (Ross, 1988).
The highest stability was obtained by refluxing the Zr solution prior to mixing with the dispersed clay, combined with a subsequent refluxing of the mixture (Burch, & Warburton, 1986; Bartley, & Burch, 1985). Intercalation of the clay by Zr polymers is very slow at room temperature. When the clay is intercalated with an unheated zirconyl chloride solution (i.e., containing only Zr tetramers) the PILC formed has the lowest surface area of all.
1.3.7.4 Effect of Drying Conditions on the Structure of Zr-PILC
Bartley, & Burch, 1985 have investigated the effect of the drying temperature on the surface area and interlayer spacing of Zr –PILC and found that a PILC with a basal spacing up to 2.5 nm, and a markedly higher surface area, was produced by replacing the water with methanol and drying at < 60°C. These polymers are believed to be mobile within the interlayer cavity of the clay prior to drying. Removing water at 110°C may cause the polymers to rearrange their positions to reduce stacking. The dried PILC
contains stacked polymer units and this results in much larger surface are being obtained.
1.3.7.5 Structure and Stability of Zr-PILC
Bartley, & Burch, 1985 found that the highest thermal stability was obtained by first refluxing the Zr solution prior to mixing with the clay and then refluxing the slurry. Table 1.2 shows the influence of the method of preparation on the thermal stability of these Zr-PILCs.
Table 1.2 Thermal stability of pillared hectorites (Occelli, & Finseth, 1986)
Heating temperature/°C Percentage retention of surface area
Al-PILC Al Zr-PILC Zr-PILC
300 100.0 100.0 100.0
400 99.0 91.0 95.4
500 95.0 91.0 96.4
600 81.0 89.5 95.3
700 44.7 44.2 77.0
Zr-PILC with good thermal stability can be prepared and the interlayer spacing, the interpillar distance, the surface area, and the Zr content can be manipulated over wide ranges by varying the method of preparation. Zr-PILCs have some interesting catalytic properties. They are active for cracking reactions, for the dehydration of methanol, and when used as a support for Cu they can be used to synthesize olefins directly from CO/H2 gas mixtures (Ross, 1988).
1.3.8 Iron Pillared Clays (Fe-PILC)
Of these publications the great majority have been concerned with Al or Zr-PILC, as these seem to offer the greatest thermal stability. In contrast, very few papers have been concerned with Fe-PILC. This is perhaps surprising given that Fe-PILC are cheaper to prepare and would not only have acidic properties, but would also contain pillars which in themselves may be catalytically active.
In order to prepare Fe-PILC it is important to understand the hydrolysis chemistry of Fe(III) solutions so that some control over the size of pillaring cations may be exercised. Although the dissolution of Fe(III) salts in water is known to result initially in simple hydrolysis products such as Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+ and Fe3(OH)45+ (Bjerrum,
Schwarzcnbach, & Sillen, 1964; Sylva, 1972), the above studies suggest that further hydrolysis leads to the formation of discrete spherical polycations. These gradually link up to give rods comprising 2 to 6 spheres. On further hydrolysis these rods appear to combine to give raft- like polycations 20x3 nm in size, which continue to grow, whole process may be brought about by either prolonged aging at room temperature, addition of base, or aging at high temperature, it is recommended that the addition of base is used, since this appears to offer the greatest degree of control over the size of the pillaring species (Warburton, 1988).
Tzou found that FeCl3, Fe(NO3)3 and Fe2(SO4)3 solutions previously aged at 25°C for
24 h gave Fe-PILC with interlayer spacing of about 0.3 nm. It was concluded that only simple hydrolysis products were responsible for pillaring in such materials (Tzou, 1983). On increasing the OH/Fe ratio from 0.0 to 1.0 interlayer spacings were found to increase (from 0.3 to1.4 nm), particularly for the chloride and nitrate systems, and thereafter increased slowly as the OH/Fe ratio was raised to 2.5. The sulphate system, in contrast, was found to give interlayer spacings of only 0.3 nm regardless of the OH/Fe ratio employed. Precipitation in this system was observed to occur at a lower OH/Fe ratio (1.5) than in the other systems (2.5). Since all precipitates were removed from the
pillaring solutions before use, the small spacings observed were attributed to pillaring by the small polycations remaining in solution (Tzou, 1983).
Tzou also showed that large interlayer spacings of 1.7 nm could be obtained using longer aging periods or higher aging temperatures, providing precipitation was avoided (Tzou, 1983).
Fe-PILC prepared from partially hydrolyzed Fe(III) solution lacked thermal stability above 300°C. This lack of thermal stability was attributed to the fact that hydroxy Fe(III) cations were unstable in water and were partially washed out during the final washing step. To overcome this problem Yamanaka et al. introduced iron pillars by exchanging Na-montmorillonite with partially hydrolyzed trinuclear aceto Fe(III) ions, which when thermally decomposed gave oxide pillars between the silicate sheets (Yamanaka, Doi, Sako, & Hattori, 1984). Using this procedure a thermally stable Fe-PILC was prepared which possessed a surface area and interlayer spacing of 280 m2 g-1 and 0.7 nm, respectively, after calcination at 500°C (Warburton, 1988).
Using a small excess of Fe3+ for the pillaring reaction (i.e. 7 mmolFe3+ /meq) surface areas of the dried materials were found to increase with OH/Fe ratio. The larger exess of Fe3+ lengthening the final washing procedure considerably, leads to a more complete pillaring, resulting in a higher surface area. As the polymeric cations produced at 75°C would be expected to lead to a larger separation of the clay layers and therefore a larger surface area (Warburton, 1988).
The surface areas of the materials calcined at 500°C generally indicate that the Fe-PILC possess good thermal stability, suffering only small reductions due to a decrease in microporosity and an increase in mesoporosity (Warburton, 1988).
1.4 Adsorption Phenomena
The use of solids for removing substances from either gaseous or liquid solutions has been widely used since biblical times. This process, known as adsorption, involves nothing more than the preferential partitioning of substances from the gaseous or liquid phase onto the surface of a solid substrate (Marczewski, 2002).
The interface of two different phases is always anisotropic. That is, in the interface the molecular interactions in one side differ from interactions in the other side. If the molecules in a phase are mobile (e.g., gas – solid or liquid – solid interfaces), then these different molecular interactions cause a concentration difference between the interface and bulk phase of mobile molecules. This type of change in concentration taking place in the interface is called adsorption. This is the reason why the interface is often referred to as the adsorption space or adsorbed phase. If the mobile molecules can penetrate into the bulk of the other phase, then this process is called absorption. It is sometimes difficult or impossible to distinguish between adsorption and absorption; it is then convenient to use the wider term sorption. The name of the phase with localized (non-mobile) molecules is the adsorbent, and the mobile molecules bound on the adsorbent surface called adsorbate. The phase with mobile molecules before the adsorption, i.e., the phase that is capable of being adsorbed is named adsorptive. So the whole process of adsorption can be expressed with the following simple and symbolic relationship (Adamczyk, 2002).
Adsorbent + Adsorptive Adsorbate (Toth, 2002).
Figure 1.11 Schematic representation of particle adsorption at solid / liquid interfaces
The solid adsorbents are often characterized by their specific surface area (as) and pore
size distribution. The value of as refers to unit mass (m) of adsorbent:
m A
a S
S =
the pore size distribution provides information on the size and amount of pores present in the solid adsorbent: Macropores are pores with widths exceeding about 50 nm. Mesopores are pores widths between 2 nm and 50 nm. Micropores are pores with widths not exceeding about 2 nm (Toth, 2002).
A large specific surface area is preferable for providing large adsorption capacity, but the creation of a large internal surface area in a limited volume inevitably gives rise to large numbers of small sized pores between adsorption surfaces. The size of the micropores determines the accessibility of adsorbate molecules to the internal adsorption
surface, so the pore size of the micropores is another important property for characterizing adsorptivity of adsorbents. Especially materials such as zeolite and carbon molecular sieves can be specifically engineered with precise pore size distributions and hence turned for a particular separation (Marczewski, 2002).
Surface polarity corresponds to affinity with polar substances such as water or alcohols. Polar adsorbents are thus called “hydrophilic” and aluminosilicates such as zeolites, porous alumina, silica gel or silica-alumina are examples of adsorbents of this type. On the other hand, nonpolar adsorbents are generally “hydrophobic”. Carbonaceous adsorbents, polymer adsorbents and silicate are typical nonpolar adsorbents. These adsorbents have more affinity with oil or hydrocarbons than water (Marczewski, 2002). The average thickness of the adsorbate is often expressed with names mono- and multilayer adsorption. In monolayer adsorption, all the adsorbed molecules are in contact with the surface of the adsorbent. In the multilayer adsorption, the adsorption space accommodates more than one layer or molecules so that not all adsorbed molecules are in direct contact with the molecules of the adsorbent (Toth, 2002). Coverage is a measure of the extent of adsorption of a species onto a surface. Usually denoted by the lower case Greek “theta”, Q. Exposure is a measure of the amount of gas which as surface has seen; more specifically, it is the product of the pressure and time of exposure (normal unit is the Langmuir, where 1 L = 10-6 Torrs).
1.4.1 Adsorption Processes
There is two type of adsorption, the first type is known as physisorption (physical adsorption) and the second one is called as chemisorption (chemical adsorption). Adsorption in which the forces involved are intermolecular forces (van der Waals forces) of the same kind as those responsible for the imperfection of real gases and the condensation vapors, and which do not involve a significant change in the electronic orbital patterns of the species involved. The term van der Waals adsorption is synonymous with physical adsorption (IUPAC Compendium of Chemical Terminology
2nd Edition, 1997). The only bonding is by weak van der Waals – type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface. A chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic at complete covalent character. This is called as chemical adsorption (Blatt, 1980, Hillier, 1995, & Velde, 1995).
Table 1.3 Typical characteristics of adsorption process
Properties Chemisorption Physisorption
Binding force
Due to chemical forces or bonding, thus this process is also called as activated adsorption.
Due to physical force of attraction, thus this process is also called as Van der Waals adsorption.
Saturation uptake Single layer phenomena Multilayer phenomena Activation energy May be involved. No activation energy
involved.
Temperature range (over which adsorption occurs)
Adsorption can take place even at higher temperatures. When temperature increases adsorption increases.
Adsorption is appreciable at lower temperature below boiling of adsorbate. When temperature increases adsorption decreases Nature of adsorption Often dissociative may be
irreversible
Non- dissociative. Reversible
Crystallographic specificity
Marked variation between crystal planes
Virtually independent of surface atomic geometry Heat of adsorption 50-100 kCal/mole 1 kCal/mole
Kinetics of adsorption
Very variable-often an activated process.
Fast- since it is a non– activated process
The problem of distinguishing between chemisorption and physisorption is basically the same as that of distinguishing between chemical and physical interaction in general.
Changes in the electronic state may be detectable by suitable means (e.g. UV, infrared or microwave spectroscopy, electrical conductivity, magnetic susceptibility).
1.4.2 Adsorption Isotherms
An adsorption isotherm for a single gaseous adsorptive on a solid is the function which relates at constant temperature the amount of substance adsorbed at equilibrium to the pressure (concentration) of the adsorptive in the gas phase (IUPAC Compendium of Chemical Terminology 2nd Edition, 1997).
The successful representation of the dynamic adsorptive separation of the solute from solution onto an adsorbent depends upon a good description of the equilibrium separation between the two phases. Adsorption equilibrium is established when the amount of solute being adsorbed onto adsorbent is equal to the amount being desorbed. At this point, the equilibrium solution concentrations remain constant. By plotting solid phase concentration against liquid phase concentration graphically, it is possible to depict the equilibrium adsorption isotherm. There are many theories relating to adsorption equilibrium.
1.4.2.1 Langmuir Isotherm
The Langmuir isotherm theory assumes monolayer coverage of adsorbate over a homogenous adsorbent surface (Langmuir, 1918). Langmuir isotherm was originally derived from adsorption kinetics by equating the rates of adsorption and desorption onto a flat surface.
1- Adsorption of adsorbate molecules takes place at well-defined adsorption sites and each site can contain only one molecule.
2- The surface is homogeneous; the energy of adsorption is equal for all adsorption sites.
Whenever a gas is in contact with a solid there will be an equilibrium established between the molecules in the gas phase and the corresponding adsorbed species (molecules or atoms) which are bound to the surface of the solid.
As with all chemical equilibrium, the position of equilibrium will depend upon a number of factors:
1. The relative stability of the adsorbed and gas phase species involved 2. The temperature of the system (both the gas and surface)
3. The pressure of the gas above the surface
In general, factor (2) and (3) exert opposite effects on the concentration of adsorbed species. We can say that the surface coverage may be increased by raising the gas pressure but will be reduced if the surface temperature is raised.
We may derive the Langmuir isotherm by treating the adsorption process as we would any other equilibrium process - except in this case the equilibrium is between the gas phase molecules (M), together with vacant surface sites, and the species adsorbed on the surface. Thus, for a non-dissociative (molecular) adsorption process we consider the adsorption to be represented by the following chemical equation:
In writing this equation we are making an inherent assumption that there are a fixed number of localized surface sites present on the surface.
We may now define an equilibrium constant (K) in terms of the concentrations of "reactants" and "products"
[
]
[
S][ ]
M M S K * − − = (1)We may also note that:
[S-M] is proportional to the surface coverage of adsorbed molecules, i.e. proportional to θ
[S -*] is proportional to the number of vacant sites, i.e. proportional to (1-θ) [M] is proportional to the pressure of gas, P
Hence, it is also possible to define another equilibrium constant, b, as given below. Rearrangement then gives the following expression for the surface coverage.
(
)
P b θ θ − = 1 (2)This is the usual form of expressing the Langmuir Isotherm.
Graphically, a plateau characterizes the Langmuir isotherm. Therefore, at equilibrium, a saturation point is reached where no further adsorption can occur. Sorption is assumed to take place at specific homogeneous sites within the adsorbent. Once a molecule occupies a site, no further adsorption can take place at that site. In Eq. (3), KL and aL are the Langmuir isotherm constants; Ce and Cs are the liquid phase
e e L L s C C a 1 K C + = (3)
The Langmuir constants, KL and aL are evaluated through linearization of Eq. (3):
e L L L s e C K a K 1 C C + = (4)
Hence by plotting Ce / Cs against Ce it is possible to obtain the value of KL from the
intercept which is (1/KL) and the value of aL from the slope, which is (aL/KL). The
theoretical monolayer capacity is Cm and is numerically equal to (KL/aL). So Eq. (4) is
rearranged as the below.
m e L m s e
C
C
a
C
1
C
C
+
=
(5)The Langmuir equation is applicable to homogeneous sorption where the sorption of each molecule has equal sorption activation energy. The equation is thermodynamically consistent and follows Henry’s Law at low concentrations. As Ce becomes lower, aL Ce
is much less than unity and Cs= KLCe, that is, analogous to Henry’s Law (Allen, Gan,
Matthews, & Johnson, 2003).
To determine if the adsorption process is favorable or unfavorable, for the Langmuir type adsorption process, the isotherm shape can be classified by a term ‘RL’, a
dimensionless constant separation factor, which is defined as below (Arslanoğlu, Kar, & Arslan, 2005).
where RL is a dimensionless separation factor. A0 is initial absorbance and aL Langmuir
constant.
The shapes of the isotherms for 0< RL < 1, RL > 1, RL = 1 and RL = 0 are favorable,
unfavorable, linear and irreversible, respectively (Weber, & Chakkravorti, 1974).
1.4.2.2 Freundlich isotherm
The Empirical Freundlich expression (Eq. (7)) is an exponential equation and therefore, assumes that as the adsorbate concentration increases so too does the concentration of adsorbate on the adsorbent surface. Theoretically, using this expression, an infinite amount of adsorption can occur (Freundlich, 1906).
Freundlich equation is based on a monolayer adsorption by the adsorbent with a heterogeneous energy distribution of active sites (Aksu & Kabasakal, 2004)
Cs = Kf Cenf (7)
In this equation Kf and nf are the Freundlich constants characteristic on the system. Kf
and nf are indicators of adsorption capacity and heterogeneity factor, respectively.
The Freundlich equation aggress well with the Langmuir expression, it does not reduce to the linear isotherm (Henry’s Law) at low surface coverage and provides no information on the monolayer adsorption capacity (Allen et. al, 2003).
Both these theories suffer from the disadvantage that equilibrium data over a wide concentration range cannot be fitted with a single set of constants (McKay, Bino, & Altemeni, 1980).
To determine the constants Kf and nf , the linear form of the equation shown below
ln Cs = ln Kf + nf ln Ce (8)
1.4.2.3 Dubinin – Radushkevich (DR) Isotherm
Langmuir and Freundlich isotherms do not give any idea about adsorption mechanism. In order to understand the adsorption type, equilibrium data is applied to following linear form of DR isotherm:
ln Csorb = ln Xm – βε2 (9)
where Csorb is the amount of ions sorbed onto the adsorbent (molg-1), Xm represents DR
monolayer capacity of the sorbent (molg-1), β a constant related to sorption energy (mol2/kJ2) and ε Polanyi sorption potential, the amount of energy required to pull a sorbed molecule from its sorption site to infinity which is equal to:
ε = RT ln (1+1/Ce) (10)
where R is the gas constant in kJmol-1K-1; T is the temperature in Kelvin and Ce is the
equilibrium concentration in solution (mol dm-3). The Polanyi adsorption theory postulates fixed volume of sorption site close to sorbent surface and existence of sorption potential over these sites. The sorption potential is related to an excess of sorption energy over the condensation energy and is independent of temperature. The plot of lnCsorb versus ε2 follows linearity. The value of Xm is determined from the
intercept and the value of β is derived from the slope. The sorption energy, E for ions onto adsorbent calculated using the expression:
2 2 / 1 − − = k E (11)
is the range of 8-16 kJmol-1 designated for ion exchange mechanism (Malik, Hasany, & Subhani, 2005). If the value of E is smaller than 8kJmol-1, this shows physical adsorption due to weak van der Waals forces (Singh & Pant, 2004).
1.4.3 Adsorption Kinetics
In order to optimize the design of an adsorption system, it is important to establish the most appropriate correlations for the equilibrium data for each system. In this respect, several kinetic models including the pseudo-first-order equation, pseudo-second-order equation, and intraparticle diffusion model are applied to find out adsorption mechanism.
1/qt = (k1/q1)(1/t) + 1/q1 Pseudo-first-order (12)
t/qt = 1/k2.q22 + (1/q2).t Pseudo-second-order (13)
qt = kp t 0.5 + C (14)
where
qt is the amount of trimethoprim adsorbed (mgg-1) on montmorillonite KSF at various
time t ,
q1 is the maximum adsorption capacity (mgg-1) for the pseudo-first order adsorption ,
k1 is the pseudo-first-order rate constant for the adsorption process (min-1),
q2 is the maximum adsorption capacity (mgg-1)for the pseudo-second-order adsorption,
k2 is the rate constant of pseudo-second-order for the adsorption (g mg-1min-1),
C is the intercept for the intraparticle diffusion model (mgg-1),
The straight-line plots of 1/qt versus 1/t for the pseudo-first-order reaction and t/qt vs
(versus) t for the second-order reaction for the adsorption have also been tested to obtain the rate parameters. The k1, k2, q1, q2 and correlation coefficients R12 and R22 of were
calculated from these plots (Özcan, & Özcan, 2004).
Adsorption kinetics are usually controlled by different mechanisms, of which the most limiting are the diffusion mechanisms, including the initial curved portion, attributed to rapid external diffusion or boundary layer diffusion and surface adsorption, and the linear portion, a gradual adsorption stage due to intraparticle diffusion starts to decrease due to the low concentration in solution as well as fever available adsorption sites. The rate-limiting step may be due to intraparticle diffusion. The rate constant for the intraparticle diffusion was obtained using the (14) equation.
C gives an idea about the boundary layer thickness. If the intraparticle diffusion is involved in the adsorption process, then a plot of the square root of time (t1/2) versus the uptake (qt) would result in a linear relationship and the particle diffusion would be the
controlling step if this line passed through the origin.
When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and the further show that the intraparticle diffusion is not the only rate-controlling step, but also other processes may control the rate of adsorption, all of which may be operating simultaneously.
The slope of the linear portion can be used to derive values for the rate parameter, kp,
for the intraparticle diffusion. The larger the intercept (C value), the greater is the boundary layer effect. (Özcan, et. al, 2004).
1.4.4 Thermodynamic Parameters of Adsorption
The thermodynamic parameters of the adsorption process are obtained from experiments at various temperatures. It is essential to clarify the change of thermodynamic parameters. Gibbs free energy change, ∆G, standard enthalpy, ∆H, and standard entropy, ∆S are estimated by applying thermodynamic equations. The Gibbs free energy change of adsorption is estimated from the following equation (Özcan, et. al, 2004).
∆G° = -RT ln Kd (15)
where
Kd = The equilibrium constant at temperature T,
R = gas constant ( 8.314 J mol-1 K-1), T = absolute temperature (K).
Kd was estimated using the following equation;
Kd = Cs / Ce (16)
where
Ce = equilibrium concentration
Cs = the amount of adsorbed
To determine the values of ∆H°and∆S°, the van’t Hoff equation is used;
ln Kd = (∆S°/R) – (∆Η°/RT) (17)
∆Η°and ∆S° can be obtained from the slope and intercept of a van’t Hoff plot of ln Kd versus 1/T (Aksu, & Kabasakal, 2004).
The negative value of the Gibbs free energy demonstrates a spontaneous and favorable adsorption process. The higher negative value reflects a more energetically favorable adsorption (Aksu, et al, 2004).
The negative value of ∆Η° indicates that the adsorption process is exothermic in nature and the negative value of ∆S° showed the decrease in degree of freedom or decrease the disorder of adsorption process. The negative value of ∆S° suggests decreased randomness at solid/ solution interface and no significant changes occur in the internal structure of the adsorbent through the adsorption (Özcan, et. al, 2004).
The activation energy, Ea, was obtained from an Arrhenius plot. Arrhenius equation is shown below;
k = A e-Ea/RT or (18)
lnk = lnA - Ea/RT (19)
where
k2 = rate constant,
A = The Arrhenius factor,
R = gas constant (8.314 Jmol-1 K-1),
T = absolute temperature (K).
The plot of lnk versus 1/T for the adsorption was applied to obtain the activation energy, Ea from the slope.
If the activation energy value is between 5-40 kJmol-1values, it is understood that the physical adsorption mechanism is occurred. When the activation value is higher than 40kJmol-1, it can be easily said that Ea represents the nature of chemical adsorption.
1.5 Trimethoprim
Trimethoprim (TMP) is among the most important antibacterial agents (synthetic antibiotics) used in human and veterinary medicine worldwide acting as an inhibitor in the chemotherapy treatment due to its antifolate effect by interaction with dihydroflate coenzymes (Florey, 1978; Hitchings, Burchall, 1965).
Figure 1.12 Chemical structure of trimethoprim
Trimethoprim is a weak base with a pKa of 7.3. It is lipophilic. The chemical designation of trimethoprim is 5-[(3, 4, 5-trimethoxyphenyl) methyl]-2, 4-pyrimidine diamine. It is a white to yellowish compound with bitter taste.
